Apparatus for increasing a polarization component, light guide unit, liquid crystal display and polarization method

ABSTRACT

The efficiency of utilizing light for obtaining polarization is enhanced by making at least part of the polarized component available that has formerly been unused. Due to a mutual difference in reflection/transmission characteristic between the s-wave component and p-wave component, the reflected light 205 (s-wave (x 1  %), p-wave (y 1  %)) and the transmitted light 206 (s-wave (x 2  %), p-wave (y 2  %)) have the respective s-wave polarized and p-wave polarized components at a different ratio (x 1  ≠x 2 , y 1  ≠y 2 ). By further changing the ratio of s-wave polarized and p-wave components by means 213 for changing the polarized direction of either the reflected light 205 or the transmitted light 206, and changing the course of light by means 212 for changing the traveling direction of light into such a direction as to enable both the reflected light 205 and the transmitted light 206 to be simultaneously utilized, light having a different ratio of s-wave and p-wave polarized components from that of the incident light 204 (s-wave (x 0  %), p-wave (y 0  %)) can be utilized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention refers to an apparatus and method for increasing thepolarization components as well as a light guide unit and liquid crystaldisplay by use thereof.

2. Related Art

Formerly, when obtaining a polarized light wave that means a light wavehaving polarization in polarized components, either s wave or p wave ofpolarized components has been absorbed by illuminating a ray ofnonpolarized light to a polarizing plate. Consequently, in principle,more than 50% of illuminated light is not made effective use of andabout 58% of illuminated light is absorbed in measured values. In aconventional liquid crystal display (LCD), not only a polarizing device(polarizing plate) for obtaining a polarized light by using theabsorption of light but also a printing dot diffusing sheet is as a ruleused, which makes further 20% of light not usable.

FIG. 15 shows LCD module 100 of a conventional LCD device. Since lightgenerated in the light source 101 exhibits approx. 96% of transmittancein the light guide plate 102, approx. 80% of transmittance in thediffusing sheet 103, approx. 42% of transmittance in the lowerpolarizing plate 104, approx. 40% of array opening ratio in the glassbase plate 105, approx. 30% of transmittance in the color filter 106,and approx. 90% of transmittance in the upper polarizing plate 107, theactually usable luminous intensity becomes 3.5% of the light generatedin the light source 101, which smallness constitutes a large hindranceto an effective use of energy. Especially, in a portable personalcomputer, it is an important theme to secure a longer serviceable timefor a fixed battery charging amount. Since a consumed power of thebacklight 108 has a large proportion in the whole consumed power, abacklight system to be used in a high-brightness andlow-power-consumption liquid crystal display (LCD) is eagerly desired.

A light energy absorbed in the lower polarizing plate 104 and the likecomes to be converted into thermal energy and this generated heatbecomes a factor for deteriorating parts of the LCD device. Inparticular, liquid crystals of STN (Super Twisted Nematic) type has aproperty of deterioration in indication quality due to heat andtherefore it is also an important theme to reduce such generation ofheat.

In this conversion of luminous energy into thermal energy, 66.4% of theluminous energy (69% of heat generation due to luminous energy) arisesfrom the absorption of light in the lower polarizing plate 104 anddiffusing sheet 103 as shown in FIG. 15.

Japanese Published Unexamined Patent Application No. 4-271324 shows onetechnique for enhancing an effective use of light, in which a lightguide comprises a plurality of laminated refracting layers havingdifferent refractive indices and a decrease in optical packet is reducedby allowing a ray of light incident from a light incident end surface tobe refracted on the boundary surface of each refractive layer and toreach the outgoing surface at a smaller angle than the critical angle,thereby promoting the light utilizing efficiency.

Also in Japanese Published Unexamined Patent Application No. 2-201316,the light utilizing efficiency of a light source is promoted by reducingthe transmission of polarizing plate to once as a result of comprising aliquid crystal cell, a light guide plate disposed therebehind, a coloredfilter provided on the light guide plate, a reflecting-plate attachedpolarizing plate provided on the rear face side of the light guideplate, and a light source provided on the lateral face of the lightguide plate.

However, either of these techniques is not different from the prior artdescribed before in that no light comprising many polarized componentsis illuminated on the polarizing plate and a polarized light is obtainedby absorbing either s wave or p wave, and does not enable at least apart of this absorbed and not-to-be-used component to be used.

Incidentally, some conventional backlight systems containing the orderof 2.7% of polarized components are seen but these have no intention topolarize the light from a light source in the backlight.

As techniques for obtaining a polarized component by usingcharacteristics of reflection or transmission without polarizing plate,there are a polarized beam splitter (PBS), transmission-type linearpolarizer, and the like. Even though able to make use of only either swave or p wave of light, or to make use of each separately, neither ofthese technique can make an effective use of both together.

Meanwhile, in a conventional transmission-type linear polarizer, becauseincident light is made to directly enter the polarizer at a Brewster'sangle, it is impossible to diffuse a converged light onto a wideoutgoing surface for outgoing. Furthermore, because a troublesometreatment is required for making the system thinner, no technique forobtaining a polarized light without using such a polarizing plate has sofar been utilized for a light guide unit of the LCD device.

SUMMARY OF THE INVENTION

By making use of a polarized component that has formerly been absorbedin the polarized plate and converting this polarized component, thepresent invention enables at least part of the polarized component thathas not formerly been made use of to be made use of and the lightutilizing efficiency to be enhanced. And, an improved aspect of thepresent invention enables nearly 100% of light utilizing efficiency tobe achieved and therefore can provide a backlight system (light guideunit) of low power consumption and high brightness.

Also, heat from the polarizing plate that has formerly been generated isreduced. Thus, the present invention can provide a backlight system(light guide unit) least likely to deteriorate in parts, coping with athermolabile LCD device.

And, an improved aspect of the present invention enables a LCD device tobe constructed without using a lower polarizing plate that has formerlybeen an essential constituent of the LCD device.

Furthermore, the present invention provides a light guide unit enablinga converged ray of light to diffuse onto a wide outgoing surface foroutgoing, or a wide ray of light to outgo toward a converged outgoingsurface and at the same time a constant polarized component to beobtained.

An apparatus for increasing polarized component in according to thepresent invention comprises: means for changing the polarizationdirection of either a ray of light reflected from or a ray of lighttransmitted through the boundary between two substances different inrefractory index for incident light comprising a first and secondpolarized components; and means for changing the traveling direction ofeither said ray of light changed in the direction of polarity or theother ray of light into such a direction as to enable these rays oflight to be used at the same time.

Means for changing the polarization direction of light includes phaseplates for a change in phase, such as quarter wave plate and half waveplate, and optical rotators for rotating a plane of polarization, suchas Faraday element, whereas means for changing the traveling directionof light includes a reflecting plate and prism sheet. And, means forchanging the polarization direction of light and at the same timechanging the traveling direction of light includes Fresnel's rhombicprism. Using all these is implied in the idea of the present invention.Furthermore, for a boundary between substances mutually different inrefractive index, an existing polarizer, such as polarizing beamsplitter or transmissive linear polarizer, is available. Thus, thepresent invention can be materialized by using an existing polarizer,means for changing the polarization direction of light and means forchanging the traveling direction of light, or by using an existingpolarizer and Fresnel's rhombic prism alone.

A light guide unit according to the present invention comprises: a mainunit body consisting of a plurality of laminated light guides and havingan outgoing surface on one side; a reflecting plate disposed on theother side of said main unit body; and means disposed between said mainunit body and said reflecting plate for changing the polarizationdirection of light.

The plurality of light guides are laminated aslant the width directionof the main unit body.

The plurality of light guides are preferably material of low internalabsorbency, such as acryl sheet, and preferably transmissive material,such as polycarbonate, polyethylene, Se and AgCl. The shape of a lightguide is not limited to plate or sheet but can be modified to thatconforming to applications, such as rod-like light guide or curvedsurface light guide. And, the plurality of light guides are not limitedto the same shape or material, but can be thought to be so designed asthick for a member requiring strength and as thin for a member requiringno strength, or to increase in the number of laminated layers whilemaintaining the strength by depositing multiple layers of materialdifferent in refractive index on a light guide of high strength. Whenusing acryl sheets as light guides, the width is preferably 0.1 to 4.0mm from the standpoint of strength and light using efficiency.

Incidentally, lamination referred to as in the present invention is notlimited to the insertion of air between the light guides, and impliesthe insertion of water or adhesive or other substances different inrefractive index from a light guide for preventing the deterioration ofa light guide due to invasion of moisture or the peeling of a lightguide.

A reflecting plate according to the present invention is the morepreferable if higher in reflectivity, and aluminum deposit sheet, silverdeposit sheet, metal foil and the like can be made mention of aspreferable.

A light guide unit according to the present invention comprises:

a main unit body consisting of a plurality of laminated light guides andhaving an outgoing surface on one side; a reflecting plate disposed onthe other side of said main unit body; and

means disposed between said main unit body and said reflecting plate forchanging the polarization direction of light. The plurality of lightguides are laminated aslant the width direction of said main unit bodyand so arranged that a ray of light is incident toward the end of areflecting plate side. A two-light backlight system with the light guideunits disposed on both side also is implied in the idea of the presentinvention.

Incidentally, as will be mentioned later, the light utilizing efficiencyimproves if the end of said reflective plate side in the plurality oflight guides is equally cut to a plane. In addition, this surface ispreferably polished to promote the reflectivity.

An improved light guide unit according to the present invention enhancesthe light utilizing efficiency by converging the rays of light incidenton the end of the reflecting plate side in parallel. Such means forconverging a ray of light includes converging means by using a lens orconvex mirror, or means by putting the end face for incident light in alight guide to a shape of convex lens, or means by combining these.

An improved light guide unit according to the present invention enhancesthe light utilizing efficiency by putting the outgoing surface of themain unit body comprising the end of a plurality of light guides into astepwise shape including a surface parallel to the outgoing anglerelated to a particular polarized component outgoing from the outgoingsurface.

An improved light guide unit according to the present invention enhancesthe light utilizing efficiency by putting the slant angle of lightguides to an angle related to Brewster's angle.

An improved light guide unit according to the present invention enhancesthe light utilizing efficiency by correcting the direction of reflectionfrom a reflecting plate. Such means are by slanting a reflecting plate,by changing the angle for outgoing, by using a Fresnel's rhombic prism,by using a prism sheet and so on. As for slanting this reflecting plate,putting the reflecting plate into a stepwise shape is also consideredfor space-saving. Furthermore, for suppressing the reflectance atreentering the light guides it is also considered to vary the travelingdirection of light stepwise by laminating materials different inrefractive index along the reflecting plate side of the main unit body.It is also possible to control the diffusion of light on the outgoingsurface by using such a configuration.

An improved light guide unit according to the present invention enhancesthe usability by correcting the outgoing direction of light on theoutgoing surface. Such means are by using a prism sheet, by machining agroove on the outgoing surface and so on. The usability is furtherenhanced by putting the shape of this prism sheet or the like into anangle related to a Brewster's angle.

A light guide unit according to the present invention includes a mainunit body comprising a main unit body consisting of a plurality oflaminated light guides and having an outgoing surface on one side,wherein the reflecting surface of plurality of light guides having theend adjacent to the outgoing surface in a shape cut to a plane.

The plurality of light guides are laminated at a slant angle related toa Brewster's angle to the width direction of said main unit body.

A light guide unit according to the present invention includes a mainunit body comprising: a main unit body consisting of a plurality oflaminated light guides and having an outgoing surface on one side; and areflecting plate disposed on the other side of said main unit body.

The plurality of light guides is laminated at a slant angle related to aBrewster's angle to the width direction of said main unit body.

A light polarizing method according to the present invention comprisesthe steps of: allowing light comprising a first and second polarizedcomponents to enter the boundary between two substances mutuallydifferent in refractive index, a part of light to be reflected andanother part of light to transmit; changing the polarization directionof said reflected light or said transmitted light; and changing thetraveling direction of said reflected light or said transmitted lightinto such a direction as to enable both said reflected light and saidtransmitted light to be used at the same time.

A light polarizing method according to the present invention comprisesthe steps of: allowing light comprising a first and second polarizedcomponents to enter the surface adjacent to an outgoing surface of amain unit body consisting of a plurality of slant, laminated lightguides and having an outgoing surface on one side; allowing lightcomprising a first and second polarized components to be reflected fromthe surface adjacent to said outgoing surface; allowing light containingmore of a first polarized component having a first outgoing directionthan a second polarized component to transmit and a part of lightcontaining more of a second component than a first component to bereflected; allowing each ray of light containing more of said secondpolarized component partly reflected than a first polarized component tooutgo from the main unit body; changing the polarization direction oflight for each ray of light containing more of said second polarizedcomponent outgoing from said main unit body than a first polarizedcomponent; reflecting each of said rays of light changed in polarizationdirection; further changing the polarization direction of said rays oflight changed and reflected into such rays of light as to contain moreof a first polarized component having a second outgoing direction than asecond polarized component; allowing each of said rays of light more ofa first polarized component having a second outgoing direction than asecond polarized component to enter said main unit body; and allowingeach of said rays of light more of a first polarized component having asecond outgoing direction than a second polarized component to transmitthrough said outgoing surface side end in each of said plurality oflight guide.

A liquid crystal display according to the present invention comprises: alight source; glass base plates for sandwiching liquid crystals; and anupper polarizing plate; means for changing the polarization direction ofeither a ray of light reflected from or a ray of light transmittedthrough the boundary between two substances different in refractoryindex for incident light comprising a first and second polarizedcomponents; and means for changing the traveling direction of eithersaid ray of light changed in the direction of polarity or the other rayof light into such a direction as to enable these rays of light to beused at the same time.

A liquid crystal display according to the present invention comprises: alight source; glass base plates for sandwiching liquid crystals; anupper polarizing plate; a main unit body consisting of a plurality oflight guides laminated aslant the width direction thereof and having anoutgoing surface on one side; a prism sheet disposed on the outgoingsurface of said main unit body; a reflecting plate disposed on the otherside of said main unit body; and means disposed between said main unitbody and said reflecting plate for changing the polarization directionof light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are to illustrate the structure of a polarizationapparatus according to the present invention;

FIGS. 3 and 4 are to illustrate an LCD device according to the presentinvention;

FIGS. 5 to 7 are to illustrate the structure of embodiments of lightguide units according to the present invention;

FIG. 8 is to illustrate the incidence of light on the end face of alight guide unit according to the present invention;

FIG. 9 is to illustrate the reflectance and transmittance observed whena ray of light enters a substance having a refractive index of 1.49 froma substance having a refractive index of 1.00 at a Brewster's angle;

FIGS. 10 to 12 are to illustrate the structure of a prism sheetaccording to the present invention;

FIGS. 13 and 14 are to illustrate a light guide unit according to thepresent invention;

FIG. 15 is to illustrate a conventional LCD device;

FIG. 16 is to illustrate the refraction of light between differentsubstances;

FIG. 17 is to illustrate a characteristic curve of reflectance observedwhen a ray of light enters a substance having a refractive index of 1.49from a substance having a refractive index of 1.00; and

FIG. 18 is to illustrate a characteristic curve of reflectance observedwhen a ray of light enters a substance having a refractive index of 1.00from a substance having a refractive index of 1.49.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Here, the operating principle of the present invention will be describedon the basis of FIGS. 1 and 2, but prior to this, to facilitate anunderstanding of operating principle, description will be made of whatchange occurs in polarized components when a ray of light transmits,refracts or is reflected on the boundary between two substances mutuallydifferent in refractive index by referring to FIGS. 16, 17 and 18.

In FIG. 16, when a ray of light 204 enters the boundary 203 between twosubstances 201 and 202 having the respective different refractiveindices n₁ and n₂, some part of light 205 is reflected and other part oflight 206 transmits if the incident angle φ₁ is equal to or less thanthe critical angle. Here, if a plane formed by the incident light at theincident point to the boundary surface is taken as an incident plane,the polarized components of the incident light 204 can be divided into ap-wave component parallel to the incident plane and a s-wave componentvertical to the incident plane.

By transforming a Maxwell equation concerning dielectrics, thetransmittance of individual polarized components, p-wave and s-wavecomponents, at this point is obtained as follows:

    Tp=sin (2φ.sub.1)×sin (2φ.sub.2)/(sin.sup.2 (φ.sub.1 +φ.sub.2)×cos.sup.2 (φ.sub.1 -φ.sub.2)

    Ts=sin (2φ.sub.1)×sin (2φ.sub.2)/sin.sup.2 (φ.sub.1 +φ.sub.2)

    n.sub.1 ×sin (φ.sub.1)=n.sub.2 ×sin (φ.sub.2),

where

Tp: transmittance for p-wave (1- reflectance Rp)

Ts: transmittance for s-wave (1- reflectance Rs)

φ₁ : incident angle of light

φ₂ : outgoing angle of light

n₁ : refractive index of substance 201 before incidence

n₂ : refractive index of substance 202 after incidence

or

    Rp=(((n.sub.1 /cos (φ.sub.1))-(n.sub.2 /cos (φ.sub.2)))/((n.sub.1 /cos (φ.sub.1))+(n.sub.2 /cos (φ.sub.2))).sup.2

    Rs=(((n.sub.1 /cos (φ.sub.1)))-(n.sub.2 ×cos (φ.sub.2)))/(n.sub.1 ×cos (φ.sub.1))+(n.sub.2 ×cos (φ.sub.2))).sup.2

As shown in FIGS. 17 and 18, a difference appears in refractive indexbetween p-wave and s-wave polarized components depending on the incidentangle φ₁ and outgoing φ₂ (the reflection/transmission characteristic forp-wave and s-wave polarized components differs). For example, when a rayof light goes from acryl of refractive index 1.49 to air of refractiveindex 1.00 (FIG. 18), the critical angle at which a total reflectionoccurs is 42.1 degree. Assuming a ray of light is incident at a smallerangle, 40 degree, an outgoing angle φ₂ becomes 77.8 degree followingSnell's raw. By substituting these values in the above introducingequation for Rs and Rp, the reflectance for s-wave and transmittance forp-wave are 35.69% and 7.98%, respectively.

And, the not reflected light transmits the light guide 202. the ratio oftransmitted light is (100-35.69=64.31)% for s-wave and (100-7.98=92.02)%for p-wave, respectively. Thus, in FIG. 16, when incident light 204comprising 100% of s-wave polarized component and 100% of p-wavecomponent goes from acryl having a refractive index of 1.49 into airhaving a refractive index of 1.00 at an angle of 40 degree, thereflected light 205 has 35.69% of s-wave polarized component and 7.98%of p-wave polarized component, whereas the transmitted light 206 has64.31% of s-wave polarized component and 92.02% of p-wave polarizedcomponent if no loss is caused in luminous energy due to a diffusedreflection on the boundary surface, an internal absorption of light inthe substances 1 and 2.

As shown in FIG. 1, a light polarizing device according to the presentinvention allows light 204 comprising a first polarized component and asecond polarized component to enter the boundary 203 between thesubstance 20having a refracactive index of n₁ and the substance 202having a refractive index of n₂, a part of light 205 to be reflected andanother part of light 206 to transmit.

At that time, a reflected part of light 205 and another transmitted partof light 206 contain a first polarized component and a second polarizedcomponent at a different ratio on account of a difference intransmission/reflection characteristic as will be described later.

And, changing the polarization of light by means 211 for changing thepolarization direction of the transmitted light 206 further changes theratio of a first polarized component and a second polarized component inthis transmitted light 206.

Furthermore, by means 212 for changing the traveling direction of light,the outgoing direction of this transmitted light 206 is changed intosuch a direction to enable both the reflected light 205 and thetransmitted light 206 to be used. Thus, at least a part of the polarizedcomponent that has formerly not been made use of becomes usable andenhancing the light using efficient becomes possible.

Here, as shown in FIG. 2, means 211 for changing the polarizationdirection of light and means 212 for changing the traveling direction oflight may be situated on either side of a reflected part of light 205 oranother transmitted part of light 206 because the feature for changingthe polarization direction and the feature for changing the travelingdirection of at least one of a reflected part of light 205 or anothertransmitted part of light 206 makes no difference on either side, andthe order of arrangement is also optional. Thus, eight combinatorialcases are possible even when restricting means 211 for changing thepolarization direction of light and means 212 for changing the travelingdirection of light respectively to one.

In addition, it is also possible to change the polarization direction ofboth a reflected part of light 205 and another transmitted part of light206 or to change the traveling direction of both, or to combine aplurality of means 211 for changing the polarization direction of lightand means 212 for changing the traveling direction of light, and it isfurther possible to change the final direction or diffusion of outgoinglight by selecting the refractive indices of substances 201 and 202,setting the incident angle, setting the angle and distance from theboundary surface of means 212 for changing the traveling direction oflight. In such cases, innumerable combinations can be considered.

To describe concrete numerical values, for example, in the case ofinverting the reflected light 205 in s-wave component and p-wavecomponent by using a half wave plate (for the purpose of description,inverting is considered, but advantages of the present invention isachieved only by changing the ratio of polarized components, forexample, through aides of a quarter wave plate) and reflecting theinverted light through a reflecting plate in such a manner as tovertically entering the boundary surface 203 (not shown), this light has7.98% of s-wave component and 35.69% of p-wave component if an energyloss on the way of propagation ignored and accordingly 72.29% of s-wavecomponent and 127.71% of p-wave component can be made use of in total ifused together with the transmitted light 206, for the transmitted lightcomprises 64.31% of s-wave component and 92.02% of p-wave component. Incontrast with the incident light 204 comprising 100% of s-wave componentand 100% of p-wave component, more of p-wave polarized component can bemade use of.

Means 211 for changing the polarization direction of light includes aphase plate for changing the phase, such as quarter wave plate or halfwave plate, an optical rotator for rotating the plane of rotation, suchas Faraday element, and the like. Converting s-wave component intop-wave component and vice versa can be made by passing through a halfwave plate once like FIG. 1 or through a quarter wave plate twice likeFIG. 2. Means 212 for changing the traveling direction of light includesa reflecting plate and prism sheet. Furthermore, means for changing thepolarization direction of light and at the same time changing thetraveling direction of light includes Fresnel's rhombic prism. Means 211for changing the polarization direction of light and means 212 forchanging the traveling direction of light in FIG. 2 can be replaced withone piece of Fresnel's rhombic prism having a particular reflectionangle.

FIG. 3 shows one embodiment of an LCD device according to the presentinvention. An LCD device 600 according to the present inventioncomprises a plurality of laminated light guides 411 having slantsurfaces 401, a reflecting plate 413, and means 412 for changing thepolarization direction of light. One embodiment of the present inventioncomprises an LCD glass panel 621, a polarizing plate 624 and a lightguide unit 626, wherein the LCD glass panel 621 comprises two glass baseplates 622 and 623 having peripheral edges sealed with edge seal 631,for example, made of epoxy resin and a liquid material is held betweenthe glass base plates 622 and 623. The light guide unit 626 comprises afluorescent lamp 414, a plurality of laminated light guides 411, areflecting plate 413, a quarter wave plate 412 and a prism sheet 629.

FIGS. 5 and 6 shows a preferred embodiment of a light guide unitaccording to the present invention. The light guide unit 400 comprises amain unit body 430 consisting of plurality of laminated light guides 411and having an outgoing surface 403 on one side; a reflecting plate 413disposed on the other side 401 of said main unit body 430; and means 412disposed between the main unit body 430 and the reflecting plate 413 forchanging the polarization direction of light.

The plurality of light guides 411 are laminated aslant the widthdirection of the main unit body 430 and are so arranged that lightenters toward the end 401 on the side of the reflecting plate 413.

Here, the plurality of light guides 411 are preferably made of amaterial having a small decrease in optical packet on the reflectingsurface, a low absorbency in the light guides and a large refractiveindex, such as acryl sheet, and can be replaced with a transmissivematerial, such as polycarbonate, polyethylene, Se and AgCl. The shape ofa light guide is not limited to plate or sheet but can be modified to ashape fit for applications, such as rod-like light guide or curvedsurface light guide. And, the plurality of light guides are not limitedto the same shape or material, but can be thought to be so designed asthick for a member requiring strength and as thin for a member requiringno strength, or to increase in the number of laminated layers whilemaintaining the strength by depositing multiple layers of materialdifferent in refractive index on a light guide of high strength. Whenusing acryl sheets as light guides, the width is preferably 0.1 to 4.0mm from the standpoint of strength and light using efficiency.

Incidentally, lamination referred to as in the present invention is notlimited to the insertion of air between the light guides, and impliesthe insertion of water or adhesive or other substances different inrefractive index from a light guide for preventing the deterioration ofa light guide due to invasion of moisture or the peeling of a lightguide.

A reflecting plate according to the present invention is the morepreferable if higher in reflectance, and aluminum deposit sheet, silverdeposit sheet, metal foil and the like can be made mention of aspreferable.

The most preferred embodiment of a light guide unit according to thepresent invention has a light source 414 on the side of end 402 andallows a ray of light to enter one and more of a plural of light guide41 1 toward surface 401.

FIG. 8 shows the entering and flow of light on the end face 402. Lightemitted from a fluorescent lamp (light source) 414 enters a light guidefrom the light guide edge.

Though rays of light enter the end face of a light guide from allangles, the rays of light after the incidence goes within ±42.1 degreefollowing Snell's law. At the point 434 in FIG. 6 mentioned later, it ispreferable to make an incident light 421 enter the surface 404 at aBrewster's angle and accordingly the light using efficiency can beenhanced by converging the incident light 421 in parallel. Means forconverging such light includes converging means by using a lens orconvex mirror, by making the end face 402 for the incidence of light ina light guide into a shape of convex lens, or by combining these.

Since light guides 411 are separated with a thin air layer 415 each fromother, a ray of light in a light guide causes a total reflection inaccordance with the relation of light guide refractive index n=1.49>airrefractive index n=1.0 at incident angles above the critical angle andpropagate to the light guide edge without loss, as shown in FIG. 8, evenif there is no converging means. Because of repeating a totalreflection, the incident light 421 has no polarization at this point.For a slant angle of 14 deg. shown in the embodiment, after repeating atotal reflection against the wedge portion (FIG. 6 should be referredto) of the light guide edge three times at the points 431, 432 and 433,the incident angle becomes smaller than the critical angle (the incidentangle decreases with smaller slant angle of a light guide) and thusoutgoes at the point of 434 outside the light guide 411.

As shown in FIG. 9, for an incident angle of 33.9 deg. at this point434, the reflectance of p-wave becomes 0. Also for the incidence from anair layer into a light guide, the reflectance of p-wave becomes 0 at anincident angle of 56.1 deg. and this angle is called Brewster's angle.The reflectance of s-wave at the Brewster's angle is 14.5%. When a rayof light enters air of refractive index n=1.00 from a light guide 411 ofrefractive index n=1.49 at the incidence angle of 33.9 deg., therefractive angle becomes 56.1 deg. in accordance with Snell's raw. Thisangle is coincident with the Brewster's angle at the outgoing from airthe a substance of refractive index n=1.49.

As seen from FIGS. 17 and 18, transmittance of p-wave at this timebecomes (100-0)×(100-0)=100%, i.e. 100% of p-wave transmits, whereasonly (100-14.5)×(100-14.5)=73.1% of s-wave transmits. Thus, every timewhen a first outgoing light 422 in FIG. 5 passes through one piece oflight guide 411, only 73.1% of s-wave transmits and (14.5+12.4)% isreflected, whereas 100% of p-wave transmits. While passing throughplate-like light guiding plates one after another, only s-wave repeatsthe reflection and the transmitted light increases the degree ofpolarization.

When a first outgoing light 422 outgoes from the upper surface (outgoingsurface) 403 after passing through about 10 layers of light guidingplates, the final transmittance at this point is (1.00)¹⁰ =100% forp-wave and (0.73)¹⁰ =4% for s-wave. In the case of putting the thicknessof a light guide into halves and using 20 laminated layers, thetransmittance of s-wave can be reduced down to (0.73)²⁰ =0.2%. In thisway, by making the thickness of a light guiding plate thinner andincreasing the number of laminated layers, the degree of polarizationcan be increased, but the thickness of a light guiding plate ispreferably 0.1 mm to 4.0 mm from the standpoint of strength in lightguiding plate when employing acryl sheets as light waves. In addition,when inserting an air layer between one light guide and another lightguide, it is preferable to make this air layer as thin as possible andalso the light guides are preferably made up of a somewhat rigidmaterial.

When converging the incident light 421 into parallel rays, a geometricalanalysis reveals the light guides 411 must slant by the following α rad!to make the incident angle at the point 434 coincident with a Brewster'sangle, where

a: (π/2-Θ₁)/2m;

Θ₁ : Brewster's angle at the incidence from a substance of refractiveindex n₁ to a substance of refractive index n₂ (=arcsin√(n₂ ² /(n₁ ² +n₂²));

n₁ : refractive index of a substance between the light guides; and

n₂ : refractive index of the light guides.

Here, m represents a natural number equal to or more than 1. When theincident light 421 is not totally reflected at the point 434 but part oflight transmits, since light containing much of p-wave componenttransmits as shown in FIG. 17, the utilizing efficiency of light becomesworse for a process of conversion into light containing much of s-wavecomponent by a later-mentioned means for changing the polarizationdirection of light (quarter wave plate) 412. Also when not totallyreflected at the point 432, the utilizing efficiency of light lowers.Thus, considering the relation with the critical angle in which theincident light 421 is totally reflected, m is preferably set as follows:

    m<(π/2-Θ.sub.1)/(Θ.sub.2 -Θ.sub.1),

where

Θ₂ : critical angle at the incidence from a substance of refractiveindex n₁ to a substance of refractive index n₂ (=arcsin (n₂ /n₁)).

When setting this m to a large value, there are advantages in that theoutgoing surface 403 opposed to the end face 402 can be set to a widearea and a thinner light guide unit can be achieved, whereas there is ademerit in requiring labors for machining. Incidentally, light guides inthe embodiment are made of acryl sheets having a refractive index of1.49. In the case of lamination in air having a refractive index of1.00, the slant angle of light guides is preferably set to 28/m deg.FIG. 7 shows one embodiment for m=3 and slant angle 405=9.3 deg.

However, machining the slant angle α of a light guide 411 to (π/2-Θ₁)/2m is difficult actually in many cases. If the reflectance of p-wavecomponent of a first out going light 422 is equal to or less than 0.1%in each layer, only 2% of loss is yielded by the relation (0.999)²⁰=0.980 after passing through 20 layers and therefore there are fewproblems. Since the tolerance of an incident angle for 0.1% or less ofreflectance of p-wave component at he point 434 is ±3 deg., thetolerance of a slant angle in a light guide is regarded as 3/2 m deg!(=π/120 m rad!).

Now, with respect to reflected s-wave, because a geometrical relationbetween the reflected s-wave and light guides is not greater than thecritical angle, the s-wave reflected from each layer of light guide goestoward the bottom face 401. This ray of light 423 passes through aquarter wave plate 412, one of means for changing the polarizationdirection of light, is reflected from a reflecting plate 413, andfurther again passes through the quarter wave plate 412. In this way, bypassing through the quarter wave plate 412 twice, the phase shifts by ahalf wavelength in total and the s-wave is converted into the p-wave.Then, the reflected light 425 enters the light guide 411 again andoutgoes from the outgoing surface 403 as a second outgoing light 427. Ifviewed as a light guide unit, the light outgoing from the outgoingsurface consists almost of p-wave component and making the polarizationaxis of light in the light guide unit coincident with the polarizationaxis of liquid crystals enables at least part of the light that hasformerly not been utilized to be utilized, and thus promoting theutilizing efficiency of light has become possible.

In an LCD device according to one embodiment of the present invention,as shown in module 600 in FIG. 4, our measurements on the lightgenerated in the light source 414 reveal that the transmittance isapprox. 96% in the light guide unit 400 and approx. 95% in the prismsheet 629 (there is no lower polarizing plate) and the array openingratio is approx. 40% in the glass base plates 622, the transmittance isapprox. 30% in the color filter 628 and approx. 90% in the upperpolarizing plate 624. Thus, 9.8% of light generated in the light source414 becomes utilizable and light utilizability increased indeed to be2.8 times in comparison with the former utilizability of 3.5% shown inFIG. 15.

Incidentally, as shown in FIGS. 5 to 7, an aspect of a plurality oflight guides 411 with the end 401 of the reflecting plate side equallycut to one plane was described, but another aspect is also possible inwhich a plurality of light guides 411 have no such shape and the mainlight guide unit is made up of commercially available acryl sheetshaving rectangular corners. In such a case, the wedge-shaped portionshown in FIG. 6 comprises an air layer. When the incident light 412 ismade to outgo from the light guide 411 to the wedge-shaped air layer,part of light is reflected from the boundary surface. If the incidentlight is not completely parallel rays, since the refractive index of thelight guide>that of air layer at the time of outgoing from the lightguide 411, diffusion of light may occur when the light outgoes from thelight guide 411 to the air layer. Furthermore, the reflection of s-wavecomponent at the point 434 shown in FIG. 6 disappears. Like this, whenselecting such a configuration, the utilizing efficiency of light lowersbut there is a benefit in that machining becomes very easy.

When applying a light guide unit according to the present invention toan actual LCD device, the first outgoing light 422 forms an angle of 20deg. and the second outgoing light 427 forms an angle of 55 deg. for thetype of a slant angle, 14 deg. in FIG. 5, whereas the first outgoinglight 422 forms an angle of 24.5 deg. and the second outgoing light 427forms an angle of 57.7 deg. for the type of a slant angle, 9.3 deg. inFIG. 7. For implementing a thinner LCD device, the direction of lightmust be corrected so that these directions 422 and 427 may be verticalto the outgoing surface 403.

As such methods, a method for disposing a prism sheet on the outgoingsurface or a method for machining a groove on the outgoing surfaceitself can be thought of.

FIGS. 10 to 12 show examples of correcting the outgoing direction oflight by using a prism sheet.

FIGS. 10 and 11 show the case of the slant angle in a light guide being14 deg., whereas FIG. 12 shows the case of the slant angle in a lightguide being 9.3 deg.

As shown in FIG. 10, a first outgoing light 422 from the outgoingsurface 403 of the light guide unit 400 enters the bottom surface 406 ofa prism sheet. Letting A be an incident angle at this time, thisincident angle A is slant against an incident angle (equal to aBrewster's angle for the most preferred embodiment) at the point 434 inFIG. 6 by the slant angle α of the light guides 411.

Thus, A can be represented as:

    A=arcsin (n.sub.1 sin Θ.sub.1)+α.

Then, letting n₃ be the refractive index of this prism sheet, theoutgoing angle B to the prism sheet can be represented in accordancewith Snell's law as:

    B=arcsin (sin A/n.sub.3).

And, the top angle C of a prism sheet having two side of a rectangulartriangle for making this light refract in the direction vertical to theoutgoing surface 403 of the light guide unit 400 and outgo is determinedin accordance with Snell's law as:

    n.sub.3 ×sin ((π/2)-B-C)=1×sin ((π/2)-C)

and therefore

    C=arctan ((n.sub.3 ×cos B-1)/sin A)

or, on eliminating B,

    C=arctan ((n.sub.3 cos (sin.sup.-1 (sin A/n.sub.3)-1)/sin A)

Furthermore, for a prism in a shape of isosceles triangle shown in FIG.11, it seems clear that the outgoing light 422 can be directed to thedirected vertical to the outgoing surface 403 by setting this top angleto double the top angle of the rectangular triangle. Incidentally, asshown in FIG. 11, for a prism sheet with the top angle between two sideof a isosceles triangle, there is a demerit in that the outgoing lightfurther is highly probable to enter the adjacent vertex, but there areadvantages in that the yield is good and the production cost is low. Inaddition, since correcting the direction of an outgoing light isperformed on the slant 437, the top angle of a rectangular or isoscelestriangle may be a curved line as shown in FIG. 12.

Incidentally, even if this corrected outgoing light does not exactlyform a rectangle to the outgoing surface 403, the slant of the order of±2 deg. is insignificant to the human eye and thus the top of a prismsheet has a tolerance of ±2 deg. (π/90 rad!) for between two sides of arectangular triangle and a tolerance of ±4 deg. (π/45 rad!) for anisosceles triangle.

Meanwhile, a design value for a prism sheet was shown in the presentembodiment, but it can be done by those skilled in the art to apply thisvalue to a groove machined on the outgoing surface itself and toidentify the shape of such a groove.

Refer to FIGS. 5 and 6 again. Since the reflecting plate 413 is arrangedin parallel with the surface 401 as shown in FIGS. 5 and 6, the outgoingangle 407 of a first outgoing light 422 and that 408 of a secondoutgoing light 427 is 20 deg. and 55 deg. respectively for the type ofslant angle, 14 deg. in FIG. 5, whereas the outgoing angle 407 of afirst outgoing light 422 and that 408 of a second outgoing light 427 is24.5 deg. and 57.7 deg. respectively for the type of slant angle, 9.3deg. in FIG. 7. Thus, even when outgoing from the outgoing surface 403(423) and converted into a p-wave component by means of a quarter waveplate, some amount of loss occurs actually because the second outgoinglight 427 does not enter a light guide 411 at a Brewster's angle.

There seem to be many cases where it is desirable to utilize a firstlight 422 and a second light 427 in the same direction. For this object,the utilizing efficiency of light can be raised with approaching of theoutgoing angle 408 of a second outgoing direction 427 to the outgoingangle 407 of a first outgoing direction 422 by correcting the directionof reflection through aids of the reflecting plate. However, it isimpossible to set the outgoing angle 408 of a second outgoing direction427 and the outgoing angle 407 of a first outgoing direction 422 equalto each other by such means alone. This is because the light 425reflected from the bottom face 401 does not transmit through a lightguide 411 but is totally reflected on account of slant arrangement oflight guides.

To solve these problems, it is also possible to laminate substancesdifferent in refractive index, make a ray stepwise refracted and causethe ray to enter the bottom face 401 (not shown). Incidentally, meansfor correcting the outgoing angle 408 of a second outgoing light 427 arenot only means by slanting a reflecting plate but also by using aFresnel's rhombic prism for changing the angle in outgoing and by usinga prism sheet. In conjunction to slanting this reflecting plate, it isalso thought of to make this plate into a stepwise shape forspace-saving.

Now, the shape of the outgoing surface of a light guide will beconsidered. If the shape of surface 409 is vertical like FIG. 7, theoutgoing light comes to enter light guides again, and accordingly theutilizing efficiency of light lowers. Thus, it is preferable to cut thesurface 409 in conformity to the outgoing angle 407 of a first outgoinglight.

Another aspect of the present invention will be described. As shown inFIGS. 13 and 14, a light guide unit according to the present aspect hasmuch the same shape as that of the above light guide unit 400, butdiffers in comprising neither reflecting plate nor means for changingthe polarization direction of light and in the requirements that the end401 of the reflecting plate side is equally cut to a plane and the slantangle of light guides is made to correspond to a Brewster's angle.

Since such a configuration alone enables a first outgoing light 422shown in FIG. 5 to be utilized and the slant angle of a light guide(angle in which the light 421 comprising both a first and secondpolarized components first enters) is set to be smaller than the angleof light guides in a conventional transmissive linear polarizer, thisaspect has a significant advantage in that the light 422 comprising moreof a first polarized component than a second polarized component can bediffused over a wider outgoing surface than a conventional transmissivelinear polarizer for outgoing (reversely, it is also possible to make aray of light enter from the outgoing surface and to converge the ray foroutgoing). Incidentally, it would be in need of no description that theshape of this light guide unit 500 has a property similar to that of theabove light guide unit 400.

Also concerning another aspect of the present invention, it is notessential to equally cut the surface 401 to a plane as with the abovelight guide unit 400, but a reflecting plate constitutes an essentialconstituent in such a case. At this time, the wedge-shaped portion shownin FIG. 6 comes to comprise an air layer as described in the light guideunit 400, but when the incident light 421 outgoes from the light guides411 to a wedge-shaped air layer, part of the light is reflected from theboundary. Unless the incident light is completely parallel rays, since(the refractive index of the light guide)>(that of air layer at the timeof outgoing from the light guide 411), diffusion of light may occur whenthe light outgoes from the light guide 411 to the air layer. Inaddition, the reflection of s-wave component at the point 434 shown inFIG. 6 disappears. Like this, when selecting such a configuration, theutilizing efficiency of light lowers but there is a benefit in thatmachining becomes very easy.

As above, the present invention achieves the utilizing efficiency oflight close to 100% by converting the polarized component that hasformerly been absorbed in a polarizing plate for obtaining thepolarization into the target polarized component, and thus can provide abacklight system of low power consumption and high brightness.

Also, the present invention solves the problems of heat from apolarizing plate that has formerly been generated, and thus can providea backlight system least likely to deteriorate in parts, coping with athermolabile LCD device.

And, the present invention enables a LCD device to be constructedwithout using a lower polarizing plate that has formerly been anessential constituent thereof.

Furthermore, the present invention can provide a light guide unitenabling a converged ray of light to diffuse onto a wide outgoingsurface for outgoing, or a wide ray of light to outgo toward a convergedoutgoing surface and at the same time a constant polarized component tobe obtained.

I claim:
 1. An apparatus for increasing either a first or secondpolarized light component, comprising:a laminated light guide having anedge surface for receiving incident light having first and secondoppositely polarized light components into a multiplicity of light guidelaminations, the laminations of the laminated light guide guiding saidreceived light and forming a multiplicity of boundary surfaces betweenlaminations for directing the first polarized light component primarilyinto a first direction and for directing the second polarized lightcomponent primarily into a second direction; means for changing thepolarization direction of light traveling in the first direction intoopposite polarization while not changing the polarization direction oflight traveling in the second direction means for changing the travelingdirection of the light traveling in either the first or second directionso as to combine the light traveling in the first and second directionsfor use together.
 2. The apparatus as set forth in claim 1 wherein saidmeans for changing the polarization direction of light is a phase plateand said means for changing the traveling direction of light is areflecting plate.
 3. A light polarizing method comprising the stepsof:allowing light comprising first and second oppositely polarizedcomponents to enter a multiplicity of laminations of a laminated lightguide through an edge thereof, the laminations of the light guideguiding said light within said laminations and forming a multiplicity ofboundary surfaces between two substances mutually different inrefractive index, a part of the light being directed by the multiplicityof boundary surfaces of the laminated light guide into a first directionand another part of the light being directed by the multiplicity ofboundary surfaces of the laminated light guide into a second direction,the light directed into the first direction primarily being the firstpolarized component and the light directed into the second directionprimarily being the second polarized component; changing thepolarization direction of the light traveling in the first direction;and changing the traveling direction of the light traveling either inthe first or the second direction so as to combine the light travelingin the first and second directions for use together.
 4. A liquid crystaldisplay comprising:a light source having first and second oppositelypolarized components; a laminated light guide unit having a multiplicityof laminations for receiving light from the light source into an edgethereof and for guiding said light within said laminations, saidlaminations of said laminated light guide forming multiple boundarysurfaces for directing light into first and second directions, the lightdirected into the first direction being primarily the first polarizationcomponent and the light directed into the second direction beingprimarily the second polarization component; glass base plates forsandwiching liquid crystals; and an upper polarizing plate; whereinsaidlight guide unit further comprises:means for changing the polarizationdirection of the light traveling in the first direction; and means forchanging the traveling direction of the light traveling either in thefirst or second direction to combine the light traveling in the firstand second directions for use together.